Strip Product Forming a Surface Coating of Perovskite or Spinel for Electrical Contacts

ABSTRACT

A strip product consists of a metallic substrate, such as stainless steel, and a coating, which in turn comprises at least one metallic layer and one reactive layer. The coated strip product is produced by providing the different layers, preferably by coating, and thereafter oxidizing the coating to accomplish a conductive surface layer comprising perovskite and/or spinel structure.

RELATED APPLICATION DATA

This application is a continuation application of U.S. patentapplication Ser. No. 11/791,321 filed Feb. 21, 2008, which is a nationalstage application of International Application No. PCT/SE05/01747 filedNov. 21, 2005, which claims priority under 37 U.S.C. §119 to SwedishApplication No. 0402936-9, filed Nov. 30, 2004, the entire contents ofeach of these applications is incorporated by reference.

FIELD OF THE DISCLOSURE

The present disclosure relates to a strip product to be used formanufacturing of electrical contacts, especially for use at hightemperatures and in corrosive environments. The strip product consistsof a metallic substrate, such as stainless steel, and a coating, whichin turn comprises at least one metallic layer and one reactive layer.The coated strip product is produced by depositing the different layersand thereafter oxidising the coating to accomplish a conductive surfacelayer comprising perovskite and/or spinel structure.

BACKGROUND

Electrical contacts are used in a large variety of environments. Severalfactors may affect the electrical contact. One example of a factor thatmay greatly affect the electrical contact is a corrosive environment. Ifthe contact material is corroded, for example by oxidation, the contactresistance is usually affected negatively. Corrosion products, like forexample electrically insulating oxides or other insulating compounds,lower the surface conductivity of the contact. This in turn results in alower efficiency of the component of which the electrical contact makesa part.

Another example of a factor that affects the material of an electricalcontact is the temperature. The contact may suffer from insufficientmechanical strength or may even weld together due to high temperature.Also, wear may affect the properties of the electrical contact.Furthermore, differences in thermal expansion between different elementsin an electrical device may cause thermal stress between the contactmaterial and its adjacent elements, especially if the contact is exposedto thermal cycling.

Naturally, high temperature in combination with a corrosive environmentcan have an even more detrimental effect on the surface conductivity ofthe contact material.

Examples of where electrical contact materials may experience highcorrosivity and high temperatures are in spark plugs, electrodes, waste,coal or peat fired boilers, in melting furnaces, in vehicles (especiallyclose to the engine), or in industrial environments etc.

Another example of an electrical contact, which is used at hightemperatures and in a corrosive environment, is interconnects for fuelcells, especially Solid Oxide Fuel Cells (SOFC). The interconnectmaterial used in fuel cells should work as both separator plate betweenthe fuel side and the oxygen/air side as well as current collector ofthe fuel cell. For an interconnect material to be a good separator platethe material has to be dense to avoid gas diffusion through the materialand to be a good current collector the interconnect material has to beelectrically conducting and should not form insulating oxide scales onits surfaces.

Interconnects can be made of for example graphite, ceramics or metals,often stainless steel. For instance, ferritic chromium steels are usedas interconnect material in SOFC, which the two following articles areexamples of: “Evaluation of Ferrite Stainless Steels as Interconnects inSOFC Stacks” by P. B. Friehling and S. Linderoth in the ProceedingsFifth European Solid Oxide Fuel Cell Forum, Lucerne, Switzerland, editedby J. Huijsmans (2002) p. 855; “Development of Ferritic Fe—Cr Alloy forSOFC separator” by T. Uehara, T. Ohno & A. Toji in the Proceedings FifthEuropean Solid Oxide Fuel Cell Forum, Lucerne, Switzerland, edited by J.Huijsmans (2002) p. 281.

In a SOFC application the thermal expansion of the interconnect materialmust not deviate greatly from the thermal expansion of theelectro-active ceramic materials used as anode, electrolyte and cathodein the fuel cell stack. Ferritic chromium steels are highly suitablematerials for this application, since the thermal expansion coefficients(TEC) of ferritic steels are close to the TECs of the electro-activeceramic materials used in the fuel cell.

An electrical contact material used as interconnect in a fuel cell willbe exposed to oxidation during operation. Especially in the case ofSOFC, this oxidation may be detrimental for the fuel cell efficiency andlifetime of the fuel cell. For example, the oxide scale formed on thesurface of the interconnect material may grow thick and may even flakeoff or crack due to thermal cycling. Therefore, the oxide scale shouldhave a good adhesion to the interconnect material. Furthermore, theformed oxide scale should also have good electrical conductivity and notgrow too thick, since thicker oxide scales will lead to an increasedinternal resistance. The formed oxide scale should also be chemicallyresistant to the gases used as fuels in a SOFC, i.e., no volatilemetal-containing species such as chromium oxyhydroxides should beformed. Volatile compounds such as chromium oxyhydroxide willcontaminate the electro-active ceramic materials in a SOFC stack, whichin turn will lead to a decrease in the efficiency of the fuel cell.Furthermore, in the case the interconnect is made out of stainlesssteel, there is a risk for chromium depletion of the steel during thelifetime of the fuel cell due to diffusion of chromium from the centreof the steel to the formed chromium oxide scale at its surface.

One disadvantage with the use of commercial ferritic chromium steel asinterconnect in SOFC is that they usually are alloyed with small amountsof aluminum and/or silicon, which will form Al₂O₃ and SiO₂,respectively, at the working temperature of the SOFC. These oxides areboth insulating, which will increase the electrical resistance of thecell, which in turn will lead to a lowering of the fuel cell efficiency.

One solution to the problems that arise when using ferritic steels asinterconnect material for SOFC are the use of ferritic steels with verylow amounts of Si and Al in order to avoid the formation of insulatingoxide layers. These steels are usually also alloyed with manganese andrare earth metals such as La. This has for instance been done in patentapplication US 2003/0059335, where the steel is alloyed (by weight) withMn 0.2-1.0%, La 0.01-0.4%, Al less than 0.2% and Si less than 0.2%.Another example is in patent application EP 1 298 228 A2 where the steelis alloyed (by weight) with Mn less 1.0%, Si less 1.0%, Al less 1.0%,along with Y less 0.5%, and/or rare earth metals (REM) less 0.2%.

In patent application U.S. Pat. No. 6,054,231 a superalloy, defined as aaustenitic stainless steel, alloys of nickel and chromium, nickel basedalloys or cobalt based alloys, is first coated with either Mn, Mg or Znand then with a thick layer, 25 to 125 μm of an additional metal fromthe group Cu, Fe, Ni, Ag, Au, Pt, Pd, Ir or Rh. The coating of a thicksecond layer of an expensive metal such as Ni, Ag or Au is not a costproductive way of protecting already relatively expensive base materialssuch as superalloys.

US2004/0058205 describes metal alloys, used as electrical contacts,which when oxidised forms a highly conductive surface. These alloys canbe applied onto a substrate, such as steel. The conducting surface isaccomplished by doping of one metal, such as Ti, with another metal,such as Nb or Ta. Furthermore, the alloys according to US2004/0058205are applied onto the surface in one step and thereafter oxidised.

None of the cited prior art provides a satisfactory electrical contactmaterial for use in corrosive environments and/or at high temperatureswhich is produced in a cost-effective manner and with a high possibilityof controlling the quality of the conductive surface.

Therefore, it is a primary object to provide a strip material with a lowsurface resistance and that is corrosion resistant, to be used in anelectrical contact.

Another object is to provide a material, which will maintain itsproperties during operation for long service lives, to be used inelectrical contacts.

A further object is to provide material that has a good mechanicalstrength, even at high temperatures, to be used as electrical contactsin corrosive environments.

Another object is to provide a cost-effective material for electricalcontacts.

SUMMARY

A strip substrate of a metallic material, preferably stainless steel,more preferably ferritic chromium steel, is provided with a coatingcomprising at least one layer of a metallic material and at least onereactive layer. In this context a reactive layer is considered to mean alayer, which consists of at least one element or compound which forms aspinel and/or a perovskite structure with the metallic material of thefirst layer when oxidised.

The strip substrate may be provided with a coating by any methodresulting in a dense and adherent coating. Coating methods may includevapour deposition, such as PVD, in a continuous roll-to-roll process.Thereafter, electrical contacts are formed of the coated strip by anyconventional forming method, such as punching, stamping or the like. Theelectrical contact, consisting of a coated strip, may be oxidised beforeassembling the electrical component of which the electrical contactmakes a part, or may be oxidised during operation.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 GDOES analysis of a 1.5 μm thick CrM coating.

FIG. 2 GIXRD diffractogram of oxidised samples with and without coating.

FIG. 3 GIXRD diffractogram of pre-oxidised samples with and withoutmetallic layer.

DETAILED DESCRIPTION

In the present disclosure the words “providing” and “provided” are to beconsidered meaning an intentional act and the result of an intentionalact, respectively. Consequently, in this context a surface provided witha layer is intended to be a result of an active action.

A perovskite and/or a spinel structure can be formed on the surfaceinstead of a “traditional” oxide on metal substrates used as electricalcontacts. The purpose of the perovskite and/or spinel structure is toaccomplish a surface with high electrical conductivity in order to havea surface with a low contact resistance. A coated strip material isproduced by providing a metallic substrate, such as stainless steel,preferably ferritic chromium steel with a chromium content of 15-30% byweight. The strip material substrate is thereafter provided with acoating consisting of at least two separate layers. One layer is ametallic layer based Al, Cr, Co, Mo, Ni, Ta, W, Zr or an alloy based onany one of these elements, preferably Cr, Co, Ni, Mo or alloys based onany one of these elements. In this context “based on” means that theelement/alloy constitutes the main component of the composition,preferably constitutes at least 50% by weight of the composition. Theother layer is a reactive layer consisting of at least one element orcompound, which forms a perovskite and/or a spinel structure with theelement/elements of the metallic layer when oxidised. The precisecomposition of the coating can be tailor-made to achieve wantedproperties, for example rate of oxide growth.

One reason for providing the surface with a coating comprising twoseparate layers, one being the metallic layer and the other being thereactive layer, is that a much more simplified production of the contactmaterial is accomplished. However, the main reason for by providing acoating with two separate layers is that it is easier to control theamount of the different elements in the perovskite/spinel, i.e. tailormake the desired composition in order to achieve the desired result.Furthermore, an excellent adhesion of the coating to the substrate canbe accomplished, thereby improving the properties of the contactmaterial and hence improving the efficiency and prolonging the servicelife in the intended application.

The reactive layer may be located on either side of the layer of ametallic material; i.e. sandwiched between the substrate and themetallic layer or, on top of the first deposited metallic layer.

According to one preferred embodiment, the metallic material consists ofessentially pure Cr or a Cr-based alloy. In this case, when the coatingis oxidised a compound with a formula of MCrO₃ and/or MCr₂O₄ is formed,wherein M is any of the previously mentioned elements/compounds from thereactive layer. The reactive layer may contain elements from Group 2A or3A of the periodic system, REM or transition metals. In this preferredembodiment the element M of the reactive layer preferably consists ofany of the following elements: La, Y, Ce, Bi, Sr, Ba, Ca, Mg, Mn, Co,Ni, Fe or mixtures thereof, more preferably La, Y, Sr, Mn, Ni, Co and ormixtures thereof. One specific example of this embodiment is one layerof Cr and the other layer being Co.

The reactive layer is obtained by preoxidation of the surface of themetallic base material according to another preferred embodiment. In thecase the metallic base material is a stainless steel, a chromium oxidewill be formed. Thereafter a layer of Ni or Co is deposited on theformed oxide according to this embodiment.

The coating may also comprise further layers. For example, the coatingmay comprise a first metallic layer, thereafter a reactive layer andfinally another metallic layer. This embodiment will further ensure agood conductivity of the surface of the electrical contact. However, dueto economical reasons the coating does not comprise more than separate10 layers, preferably not more than 5 separate layers.

The thickness of the different layers are usually less than 20 μm,preferably less than 10 μm, more preferably less than 5 μm, mostpreferably less than 1 μm. The thickness is preferably adapted to therequirements of the intended use of the electrical contact. According toone embodiment the thickness of the reactive layer is less than that ofthe metallic layer. This is especially important when the reactive layercomprises elements or compounds that upon oxidation themselves formnon-conducting oxides. In this case it is important that essentially thewhole reactive layer/layers are allowed to react and/or diffuse into themetallic layer at least during operation of the electrical contact, sothat the conductivity of the contact during operation is not affectednegatively.

The thickness of the strip substrate may be 5 mm or less, preferablyless than 2 mm and most preferably less than 1 mm. The width of thestrip may be up to 1200 mm, preferably at least 100 mm. Naturally, thethickness has to be adapted to the requirements of the final applicationof the electrical contact. One advantage of making a coated stripaccording to the present disclosure is that both small and largeelectrical contacts can be formed from the strip, for example bystamping or punching. This makes the manufacturing process morecost-effective. However, in some cases other forms of substrate might beapplicable. One example where the substrate advantageously is in theform of a bar is in the application of support bars in electrochemicalcells. The substrate may also be in form of a wire or tube if theintended use of the electrical contacts so requires.

The coated strip may be produced in a batch like process or continuousprocess. However, for economical reasons, the strip may be produced inlengths of at least 100 m, preferably at least 1 km, most preferably atleast 5 km, in a continuous roll-to-roll process. The coating may beprovided onto the substrate by coating with the metallic layer and thereactive layer. However, according to an alternative embodiment thecoating may also be provided by pre-oxidation of the substrate to anoxide thickness of at least 50 nm and thereafter coating with theadditional layer. The coating is thereafter oxidised further as toachieve the spinel and/or perovskite. This alternative embodiment ofproviding the coating onto the base material is especially applicablewhen the base material is ferritic chromium steel, such as the oxideformed on the surface is a chromium based oxide.

The coating may be performed with any coating process that generates athin dense coating with good adhesion to the underlying material, i.e.the substrate or an underlying coating layer. Naturally, the surface ofthe strip has to be cleaned in a proper way before coating, for exampleto remove oil residues and/or the native oxide layer of the substrate.According to one preferred embodiment, the coating is performed by theusage of PVD technique in a continuous roll-to-roll process, preferablyelectron beam evaporation which might be reactive or even plasmaactivated if needed.

Furthermore, the strip may be provided with a coating on one side or onboth sides. In the case the coating is provided on both surfaces of thestrip, the composition of the different layers on each side of the stripmay be the same but may also differ, depending on the application inwhich the electrical contact will operate. The strip may be coated onboth sides simultaneously or one side at a time.

Optionally, the coated strip is exposed to an intermediatehomogenisation step as to mix the separate layers and accomplish ahomogenous coating. The homogenisation can be achieved by anyconventional heat treatment under appropriate atmosphere, which could bevacuum or a reducing atmosphere, such as hydrogen or mixtures ofhydrogen gas and inert gas, such as nitrogen, argon or helium.

The coated strip is thereafter oxidised at a temperature above roomtemperature, preferably above 100° C., more preferably above 300° C., sothat a perovskite and/or a spinel structure is formed on the surface ofthe strip. Naturally, the coating thickness will increase when thecoating is oxidised due to the spinel and/or perovskite formation. Theoxidation may result in a total oxidation of the coating or a partiallyoxidation of the coating, depending on for example the thickness of thelayers, if the coating is homogenised, and time and temperature of theoxidation. In either case, the different layers of the coating areallowed to at least partially react and/or diffuse into each other, ifthis is not done by an intermediate homogenisation step. The oxidationmay be performed directly after coating, i.e. before the formation ofthe electrical contact, after formation to the shape of the finalapplication, i.e. the manufacturing of the electrical contact from thecoated strip, or after the electrical appliance, for example a fuelcell, has been assembled, i.e. during operation.

The purpose of accomplishing a perovskite and/or a spinel structure onthe surface of the strip is that the formed perovskite and/or spinel hasa much lower resistance compared to traditional oxides of the elementsof the metallic layer. This will in turn lead to a lower contactresistance of the electrical contact and therefore also a betterefficiency of the component of which the electrical contact makes apart. For example, the resistivity of Cr₂O₃ at 800° C. is about 7800Ω·cm while the resistivity of for example La_(0.85)Sr_(0.15)CrO₃ isseveral orders of magnitude lower, namely about 0.01 Ω·cm.

Also, in the case of chromium containing ternary oxides such as spineland perovskites it is believe that these oxides are much less volatilethan pure Cr₂O₃ at high temperatures.

Furthermore, by providing a perovskite and/or spinel structure on thesurface of a substrate such as stainless steel the electrical contactwill have good mechanical strength and is less expensive to manufacturethan for example electrical contacts made entirely from a perovskiteand/or spinel based ceramics.

Also, in the case where the substrate is a stainless steel the chromiumdepletion of the substrate is inhibited since the metallic layer willoxidise long before chromium of the substrate, this is especiallypronounced when the metallic layer is Cr or a Cr-based alloy. Therefore,the corrosion resistance of the substrate will not be reduced duringoperation.

Moreover, according to one optional embodiment Mn and/or REM from thesubstrate is allowed to diffuse into the coating. This may in some casesfurther promote the formation of a perovskite or spinel structure on thesurface. Even small contents of Mn and/or REM of the substrate mayaffect the formation of the final structure. The content of Mn in thesubstrate is preferably 0.1-5 wt %, the content of REM is preferably0.01-3 wt % and the content of Cr in the substrate is preferably 15-30wt %. Naturally, the needed content of Mn and/or REM depends on thethickness of the coating. Thicker coatings need higher contents of Mnand/or REM. For example, if the coating is less than 2 μm a content of0.1-1 wt % Mn is sufficient as to achieve the desired result.

In some cases it might be applicable to have one surface of theelectrical contact conductive while the other should be non-conductive,i.e. isolating. In these cases the coating as described previously maybe applied to one surface and an electrically isolating material such asAl₂O₃ or SiO₂ may be applied to the other surface. This may be donein-line with the electrically conductive coating. According to oneexample a coating comprising one metallic layer and one reactive layeris provided to one surface of the strip and a metal which will form aninsulating layer when oxidised, such as for example Al, is be applied tothe other surface of the strip. The coated strip is thereafter oxidisedresulting in one conductive surface and one insulating surface.

As an alternative to the above-described, one might apply the coating byother processes, for example by co-evaporation of the differentcomponents of the coating or by electrochemical processes.

Examples of coated strips will now be described. These should not beseen as limiting but merely of illustrative nature.

Example 1

A stainless steel substrate is coated with a coating consisting of ametallic layer and a reactive layer. The metallic layer is a Cr or aCr-based alloy. The reactive layer in this case includes transitionmetals, such as Ni, Co, Mn and/or Fe, if the oxide should receive aspinel structure. If a perovskite structure is desired, the reactivelayer contains elements from Group 2A or 3A of the periodic system, orREM. Preferably, the reactive layer contains Ba, Sr, Ca, Y, La and/orCe. If a mixed structure including both a spinel and a perovskitestructure, the reactive layer may contain elements from Group 2A or 3Aof the periodic system, or REM along with transition metals.Alternatively, Mn and/or REM are allowed to diffuse from the substrate.

The coating is optionally homogenised and thereafter oxidised so as toform the desired structure on the surface. This results in a very lowsurface resistance of the strip substrate. Also, the Cr-oxides MCrO₃and/or MCr₂O₄ formed during oxidation are less volatile than pure Cr₂O₃at high temperatures. This results in a coated strip that is highlysuitable to be used as contact material in corrosive environments evenat high temperatures, for example as interconnects in Solid Oxide FuelCells.

Example 2

A 0.2 mm thick strip substrate of a ferritic chromium stainless steelwas coated. The coating was homogenised so as to achieve a CrM layerwherein M is a mixture of La and Mn. The concentration of Cr in thecoating is approximately 35-55 wt %, while the concentration of Mn isapproximately 30-60 wt % and the concentration of La is 3-4 wt %.

The surface was analysed by Glow Discharge Optical Emission Spectroscopy(GDOES). Using this technique, it is possible to study the chemicalcomposition of the surface layer as a function of the distance from thesurface. The method is very sensitive for small differences inconcentration and has a depth resolution of a few nanometres. The resultof the GDOES analysis of a 1.5 μm thick CrM surface alloying layer isshown in FIG. 1.

Example 3

Two samples of a ferritic chromium steel with the nominal composition,by weight max 0.050% C; max 0.25% Si; max 0.35% Mn; 21-23% Cr; max 0.40%Ni; 0.80-1.2% Mo; max 0.01% Al; 0.60-0.90% Nb; small additions of V, Tiand Zr and natural occurring impurities were manufactured. One of thesamples was coated with a 0.1 μm thick cobalt layer and a 0.3 μm thickchromium layer. The samples were oxidised in air at 850° C. for 168hours prior to the analysis. The samples were analysed by GrazingIncidence X-Ray Diffraction (GIXRD) with an incidence angle of 0.5°, seeFIG. 2. It should be pointed out that GIXRD is a surface sensitivediffraction method and only the crystalline phase of the top layer onthe oxidised steel is analysed. Any crystalline phase present under thetop layer which is not reached by the grazing X-rays will not be seen inthe diffractogram. The amount of spinel vs. chromium oxide formed in thetop layer of the oxide scale of each sample were compared by measuringthe peak to bottom intensity of the Cr₂O₃ (Eskolaite) reflection at 2theta=36.7° (3) and dividing it by the intensity of the spinelreflection at 2 theta≅=45° (4). The ratio of Eskolaite/spinel for theuncoated oxidised samples was 9.9 while for the coated sample the ratiowas 1.0. This could be interpreted as a ten-fold increase of spinelstructure in the surface oxide scale formed. In FIG. 2 the (1)diffractogram is the uncoated sample oxidised in air for 168 hours at850° C. and the (2) diffractogram is the coated sample oxidised in airfor 168 hours at 850° C.

Example 4

Three samples of a ferritic chromium steel with the nominal composition,by weight max 0.050% C; max 0.25% Si; max 0.35% Mn; 21-23% Cr; max 0.40%Ni; 0.80-1.2% Mo; max 0.01% Al; 0.60-0.90% Nb; small addition of V, Tiand Zr and normally occurring impurities were manufactured. Two of thesamples were pre-oxidised in air to get a 100 nm thick oxide scale. Thepre-oxidised samples were thereafter coated with a metallic layer. Themetallic layer on sample 2 was a 300 nm thick Ni layer and on sample 3 a300 nm thick Co layer. All three samples were then further oxidised inair at 850° C. for 168 hours prior to the analysis. The samples wereanalysed by Grazing Incidence X-Ray Diffraction (GIXRD) with anincidence angle of 0.5°, see FIG. 3. It should be pointed out that GIXRDis a surface sensitive diffraction method and only the crystalline phaseof the top layer on the oxidised steel is analysed. Any crystallinephase present under the top layer which is not reached by the grazingX-rays will not be seen in the diffractogram. The amount of spinel vs.chromium oxide formed in the top layer of the oxide scale of each samplewere compared by measuring the peak to bottom intensity of the Cr₂O₃(Eskolaite) reflection at 2 theta=36.7°. (4) and dividing it by theintensity of the spinel MCr₂O₄ reflection at 2 theta≅=45°. (5). Theratio of Cr₂O₃/MCr₂O₄ for the uncoated oxidised samples was 9.9 whilefor the pre-oxidised sample with the Ni layer the ratio was 1.26 and forthe pre-oxidised sample with the Co layer the ratio was 0.98. Thisindicating an 8.5, respective 10 folded increase of spinel structure inthe formed oxide scale. Interesting to note here is that the nickellayer does not only form more spinel oxide in the scale but also NiO isformed when the sample has been oxidised (6). In FIG. 3 the (1)diffractogram is the uncoated sample oxidised in air for 168 hours at850° C., the (2) diffractogram is the pre-oxidised sample with a Nilayer sample oxidised in air for 168 hours at 850° C. and the (3)diffractogram is the pre-oxidised sample with a Co layer sample oxidisedin air for 168 hours at 850° C.

What is claimed is:
 1. Strip product to be used as electrical contactconsisting of a stainless steel base material and a coating provided onthe stainless steel base material, wherein the coating comprises atleast one metallic layer, and at least one reactive layer which forms aspinel and/or perovskite structure with the at least one metallic layerwhen oxidised, wherein a total thickness of the at least one reactivelayer is less than a total thickness of the at least one metallic layer,wherein a composition of the at least one metallic layer includes Co ora Co-based alloy, and wherein a composition of the at least one reactivelayer includes Ce or a Ce-based alloy, wherein the at least one reactivelayer is located between the at least one metallic layer and thestainless steel base material.
 2. Strip product according to claim 1,wherein each of the layers is less than 20 μm thick.
 3. Strip productaccording to claim 1, wherein the composition of the stainless steelbase material comprises Mn in an amount of 0.1-5% by weight and/or REMin an amount of 0.01-3% by weight.
 4. Strip product according to claim1, wherein the at least one metallic layer comprises at least twoseparate metallic layers in addition to the at least one reactive layer.5. Strip product according to claim 4, wherein a composition of each ofthe at least two separate metallic layers includes Co or the Co-basedalloy.
 6. Strip product according to claim 1, wherein the coatingconsists of one metallic layer and one reactive layer.
 7. Strip productaccording to claim 1, wherein a total number of the at least onereactive layer and the at least one metallic layer in the coating is nomore than
 10. 8. Strip product according to claim 7, wherein the totalnumber of the at least one reactive layer and the at least one metalliclayer in the coating is no more than
 5. 9. Strip product to be used aselectrical contact consisting of: a stainless steel base material; and acoating provided on the stainless steel base material, wherein thecoating comprises at least one metallic layer of Co or a Co-based alloy,and at least one reactive layer containing at least one element orcompound which forms a spinel and/or perovskite structure with the Co ora Co-based alloy of the at least one metallic layer of Co or a Co-basedalloy when oxidized, wherein the at least one element or compound whichforms a spinel and/or perovskite structure is Ce or a Ce-based alloy,wherein a thickness of the at least one reactive layer is less than athickness of the at least one metallic layer, wherein each of the atleast one reactive layer and the at least one metallic layer is lessthan 1 μm thick, and wherein the at least one reactive layer is locatedbetween the at least one metallic layer and the stainless steel basematerial.
 10. Strip product according to claim 9, wherein each of thelayers is less than 20 μm thick.
 11. Strip product according to claim 9,wherein the at least one metallic layer comprises at least two separatemetallic layers in addition to the at least one reactive layer. 12.Strip product according to claim 9, wherein the composition of thestainless steel base material comprises Mn in an amount of 0.1-5% byweight and/or REM in an amount of 0.01-3% by weight.
 13. Strip productaccording to claim 9, wherein the coating consists of one metallic layerand one reactive layer.
 14. Strip product according to claim 9, whereina total number of the at least one reactive layer and the at least onemetallic layer in the coating is no more than
 10. 15. Strip productaccording to claim 14, wherein the total number of the at least onereactive layer and the at least one metallic layer in the coating is nomore than
 5. 16. Strip product to be used as electrical contactconsisting of a stainless steel base material and a coating provided onthe stainless steel base material, wherein the coating comprises atleast one metallic layer, and at least one reactive layer which forms aspinel and/or perovskite structure with the at least one metallic layerwhen oxidised, wherein a total thickness of the at least one reactivelayer is less than a total thickness of the at least one metallic layer,wherein a composition of the at least one metallic layer consists of Coor a Co-based alloy, and wherein a composition of the at least onereactive layer consists of Ce or a Ce-based alloy, wherein the at leastone reactive layer is located between the at least one metallic layerand the stainless steel base material.